In offset lithography, an image is present on a plate as a pattern of ink-accepting and ink-repellent surface areas. In a typical sheet-fed press system, the plate is mounted to a plate cylinder, is inked and makes contact with the compliant surface of a blanket cylinder which, in turn, applies the image to paper sheets pinned to an impression cylinder, which brings the sheets into contact with the blanket cylinder.
Traditionally, the plates for an offset press have usually been produced photographically. More recently, however, a number of electronic alternatives have been developed, some of which can be used on press. With such systems, a digitally controlled imaging device on a write head alters the ink-receptivity of a plate blank mounted to a cylinder in a pattern representative of the image to be printed. Such imaging devices include sources of electromagnetic-radiation pulses such as lasers and spark or ion discharge sources which physically alter the plate blank, thereby producing "dots" which collectively form a desired image; see e.g., U.S. Pat. Nos. 5,148,746; 5,385,092; 5,379,698 and 5,249,525.
One such plate, described in U.S. Pat. No. 5,148,746, comprises a silicon surface layer which is ink repellent, a thin intermediate layer of metal such as aluminum or titanium and an ink-receptive base or substrate of a film material such polyester marketed under the trademark Mylar (duPont) or Melinex (ICI). That plate may be imaged by delivering spark or ion discharge pulses to the plate which burn through the silicone layer and vaporize or ablate the metal intermediate layer. The imaging pulses effectively detach the ink repellent silicon layer from the ink receptive substrate, facilitating easy removal of the silicone at the image spots. After the plate is cleaned, the plate will carry an image composed of a multiplicity of image dots which have relatively regular boundaries on all sides and are substantially free of debris.
Another plate of this general type, described in U.S. Pat. No. 5,385,092 and which is imaged photographically, comprises an ink repellent surface layer, e.g., silicone, coated onto a photosensitive layer, which is itself coated onto a substrate of suitable stability (e.g., an aluminum or titanium sheet). Upon exposure to actinic radiation, the photosensitive layer cures to a state that destroys its bonding to the surface layer. After exposure, a treatment is applied to deactivate the photoresponse of the photosensitive layer in unexposed areas and to further improve anchorage of the surface layer to these areas. Immersion of the exposed plate in developer results in dissolution and removal of the surface layer at those portions of the plate surface that have received radiation, thereby exposing the ink-receptive, cured photosensitive layer. That type of plate also requires post imaging cleaning.
Various approaches have been suggested for removing plate debris produced in the course of platemaking and specifically in connection with platemaking processes involving ablation or removal of one or more material layers of the plate. One such cleaning system is disclosed in the above U.S. Pat. No. 5,148,746. Basically, that system comprises a rotating brush mounted on the writing head which can be moved into contact with the surface of a lithographic plate mounted to a plate cylinder. That system also delivers a cleaning fluid to the brush in order to assist the debris-removal action thereof.
While that prior plate cleaning apparatus operates satisfactorily in many respects, it is relatively slow because the brush only cleans a relatively small area of the plate at any given time. In other words, the brush head must be moved along the entire length of the plate cylinder in order to clean the entire surface area of the plate.
Furthermore, with some plate constructions, that prior cleaning apparatus is not sufficiently effective in removing all of the debris from the plate. This is partly due to the construction of the brush head itself which is simply comprised of bristles and partly due to the fact that insufficient provision is made for removing debris from the brush.